Low rates and mid rates is where people will spend most of their time. It takes time (players will be playing longer on your server, thus a higher chance of donations to the server, and resulting into high population) to play the game.
Not too much with high rate, but PvP is what its for right? PvP for maybe 10~30 minutes and then logging out to play something else.
L/M rates will promote guild recruitment, leveling parties, battleground enjoyment, and a more memorable experience.